1. Field of the Invention
This invention relates to leuco dyes, and, more particularly, to yellow and magenta chromogenic leuco dyes that are suitable for use in photothermographic imaging systems.
2. Discussion of the Art
Silver halide photothermographic imaging materials (i.e., heat developable photographic materials) and that are classified as "dry silver" compositions or emulsions, and are processed with heat and without liquid development and have been known in the art for many years. Such materials comprise (1) a light-insensitive, reducible silver source, (2) a light-sensitive material that generates atomic silver when irradiated, and (3) a reducing agent for the reducible silver source. The light-sensitive material is generally photographic silver halide, which must be in catalytic proximity to the light-insensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source. It has long been understood that atomic silver (Ag.degree.) is a catalyst for the reduction of silver ions, and the light-sensitive photographic silver halide may be placed into catalytic proximity with the light-insensitive, reducible silver source in a number of different fashions, such as partial metathesis of the reducible silver source with a halogen-containing source (see, for example, U.S. Pat. No. 3,457,075), coprecipitation of silver halide and reducible silver source material (see, for example, U.S. Pat. No. 3,839,049), blending, and other methods that intimately associate the light-sensitive photographic silver halide and the light-insensitive, reducible silver source.
The light-insensitive, reducible silver source is a material that contains silver ions. The preferred light-insensitive reducible silver source comprises silver salts of long chain aliphatic carboxylic acids, typically having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates have been proposed, and U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as light-insensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image generally is not visible by ordinary means and the light-sensitive emulsion must be further processed in order to produce a visible image. The visible image is produced by the reduction of silver ions, which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms, i.e. the latent image.
As the visible image is produced entirely by silver atoms (Ag.degree.), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is desirable in order to reduce the cost of raw materials used in the emulsion.
One conventional way of attempting to increase the maximum image density of photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming materials in the emulsion. Such dye-forming materials include leuco dyes, which are the reduced form of a color-bearing dye. Upon imaging, the leuco dye is oxidized, and the color-bearing dye and a reduced silver image are simultaneously formed in the exposed region. In this way a dye enhanced silver image can be produced, as shown for example in U.S. Pat. Nos. 3,531,286; 4,187,108; 4,426,441; 4,374,921; and 4,460,681. However, when the reactants and reaction products of photothermographic systems that contain leuco dyes remain in contact after imaging, several problems can result. For example, thermal development often forms turbid and hazy color images because of dye contamination of the reduced metallic silver image on the exposed area of the emulsion. In addition, the resulting prints tend to develop color in unimaged background areas. This "background stain" is caused by slow reaction between the leuco dye and reducing agent during storage.
Multicolor photothermographic imaging articles typically comprise two or more monocolor-forming emulsion layers (often each emulsion layer comprises a set of bilayers containing the color-forming reactants) maintained distinct from each other by barrier layers. The barrier layer overlaying one photosensitive, photothermographic emulsion layer typically is insoluble in the solvent of the next photosensitive, photothermographic emulsion layer. Photothermographic articles having at least 2 or 3 distinct color-forming emulsion layers are disclosed in U.S. Pat. Nos. 4,021,240 and 4,460,681. Various methods to produce dye images and multicolor images with photographic color couplers and leuco dyes are well known in the art as represented by U.S. Pat. Nos. 4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747 and Research Disclosure 29963.
A common problem that exists with these photothermographic systems is the instability of the image following processing. The photoactive silver halide still present in the developed image may continue to catalyze print-out of metallic silver even during room light handling causing a strong increase of fog after development. This is also increased by the presence of oxygen in the air which causes the oxidation of leuco dyes. For example, U.S. Pat. Nos. 4,670,374 and 4,889,932 describe photothermographic materials containing oxidable leuco phenazine, phenoxazine or phenothiazine dyes useful to give color photothermographic images. Unfortunately they are subjected to aerial oxidation, which causes increasing fog after development.
Another problem is the lack of stability of the leuco dyes before exposure: in fact, in many cases, it is not possible to obtain any images because the leuco dye reacts in a non-image-wise way before exposure. The consequence of this non-image-wise reaction is the absence of sensitometric effects. This means that there is no difference in the print-out between the parts that should have produced an image and the parts that should not have produced any image. European Patent Application No. 35,262, and PCT Patent application No. WO 90-00,978 describe, respectively, non-silver copy materials and non-silver heat-sensitive materials both having leuco dyes with the same --SO.sub.2 -- protecting group. These leuco dyes are useful in heat-sensitive materials. They are not useful in photothermographic materials because they do not react image-wise to give a dye image. In fact, when the material containing such leuco dyes is exposed and developed according to the usual process for photothermographic materials, it does not present any sensitometric effects.
Thus, there exists a need to have useful leuco dyes for photothermographic materials which are stable enough not to be oxidised by contact with air or by simple heating, and which limit fog formation after development to the simple print-out due to the presence of photosensitive silver halide. They also must react image-wise to provide a good dye image.
British Patent No. GB 1,417,586 describes the preparation of oxichromic compounds containing a reduced azomethine linkage. Such compounds produce upon chromogenic oxidation a chromophore useful in colour photographic systems, particularly in silver halide transfer materials. These oxichromic compounds may have a group which prevents oxidation of the N atom of the azomethine linkage and which hydrolizes off in alkaline solution and, in addition, they have a hydroquinone moiety in their structures. They are hence different from the compounds of the present invention and are used for a different purpose.
A number of methods have been proposed for obtaining colour images with dry silver systems. Such methods include incorporated coupler materials, e.g., a combination of silver benzotriazole, well known magenta, yellow and cyan dye-forming couplers, aminophenol developing agents, a base release agent such as guanidinium trichloroacetate and silver bromide in poly(vinyl butyral); a combination of silver bromoiodide, sulphonamidophenol reducing agent, silver behenate, poly(vinyl butyral), an amine such as n-octadecylamine and 2-equivalent or 4-equivalent cyan, magenta or yellow dye-forming couplers; incorporating leuco dye bases which oxidizes to form a dye image, e.g., Malechite Green, Crystal Violet and pararosaniline; a combination of in situ silver halide, silver behenate, 3-methyl1-phenylpyrazolone and N,N-dimethyl-p-phenylenediamine hydrochloride; incorporating phenolic leuco dye reducing agents such as 2-(3,5-di-tert-butyl-4-hydroxyphenyl)-4,5-diphenylimidazole, and bis-(3,5-di-t-butyl-4-hydroxyphenyl)phenylmethane, incorporating azomethine dyes or azo dye reducing agents; silver dye bleach process, e.g., an element comprising silver behenate, behenic acid, poly(vinyl butyral), poly(vinyl-butyral)peptized silver bromoiodide emulsion, 2,6-dichloro-4-benzenesulfonamidophenol, 1,8-(3,6-diazaoctane)bis-isothiuronium-p-toluene sulfonate and an azo dye which was exposed and heat processed to obtain a negative silver image with a uniform distribution of dye which was laminated to an acid activator sheet comprising polyacrylic acid, thiourea and p-toluene sulfonic acid and heated to obtain well defined positive images; and incorporating amines such as amino acetanilide (yellow dye-forming) 3,3'-dimethoxybenzidine (blue dye-forming) or sulfanilanilide (magenta dye forming) which react with the oxidized form of incorporated reducing agents such as 2,6-dichloro-4-benzene-sulfonamido-phenol to form dye images. Neutral dye images can be obtained by the addition of amines such as behenylamine and p-anisidine.
Leuco dye oxidation in such silver halide systems are disclosed in U.S. Pat. Nos. 4,021,240, 4,374,821, 4,460,681 and 4,883,747.